In recent years, there is a strong demand for high resolution, low cost, and miniaturization for an imaging device mounted in a mobile phone, a personal computer (PC), and the like.
As the cell pitch in an imaging element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor) image sensor, is significantly decreased, high imaging performance capable of suppressing optical aberrations, especially axial chromatic aberration, is requested for an optical system compared with a normal optical system.
In addition, in response to price requests, a technique of manufacturing a large number of lenses at the same time in the form of a wafer in order to reduce the cost is known.
As a typical example, a technique disclosed in PTL 1 is known.
The technique disclosed in PTL 1 is called a hybrid method.
In the hybrid method, a number of lenses are formed on the wafer-shaped glass substrate, and the imaging element wafer and the lens elements are bonded in a wafer state. Then, these are separated into pieces to manufacture a large number of camera modules at the same time.
The hybrid method is advantageous in that an IR cut filter or a diaphragm can be formed on a glass wafer and accordingly these separate components are not necessary unlike the related art, and a large number of finished products can be manufactured at the same time, and accordingly the number of assembly steps per product is reduced and products can be manufactured inexpensively.
In the former case, since no separate IR cut filter is necessary, there is also an advantage in that the back focus of the lens can be short. Therefore, a more flexible optical design is possible.
As a specific example of the optical design used in these techniques, that disclosed in PTL 2 is known.
In this technique, a 2-group configuration is adopted to meet high imaging performance and the short optical length.
In addition, in order to further improve the characteristics, it is possible to adopt a doublet structure in which lens resins overlap each other.
As such an example, the technique disclosed in PTL 3 is known. PTL 3 discloses realizing a high NA by disposing replica lenses in one place so as to overlap each other.